Cardiac surgery is generally followed by a period of routine ventilator support. When the patient seems hemodynamically stable and relatively alert following surgery, respiratory adequacy is tested by the weaning trial. In this study, physiological and clinical prediction of postoperative respiratory adequacy, including values of pulmonary function tests, were examined in an attempt to identity those few variables which predicted the outcome of the ventilator weaning trial following surgery. Our series comprised 27 patients who underwent elective open intracardiac operations at the Department of Thoracic and Cardiovascular Surgery, School of Medicine, Kyungpook National University, from October 1979 to July, 1980. The pulmonary function tests performed on all patients included the following; forced vital capacity [FVC], forced expiratory volume [FEV1.0], forced expiratory flow [FEF 25--75~], residual volume [RV], and functional residual capacity [FRC], measured with a helium dilution technique. Of our 27 patients, 8 were successfully weaned within 20 hours of operation. All patients with cyanotic heart diseases or acquired heart diseases were unsuccessfully weaned. The bypass time in the successful weaning group was shorter in the mean value [82.8 minutes]than in the unsuccessful weaning group [120.5 minutes]. There was a relatively significant difference in the mean values for the two groups in arterial pressure, bleeding amounts and FiO2 among the postoperative monitoring variables, and in forced vital capacity [FVC]. The postoperative clinical assessments appeared vague but corresponded reasonably well to appraisal of success in weaning, especially in variables of cough and self-respiration efforts. [KTCS 1981;3:187-194]